U.S. patent number 3,876,736 [Application Number 05/289,993] was granted by the patent office on 1975-04-08 for method of molding synthetic resins through high-pressure fluid cross-linking process and relevant apparatus.
This patent grant is currently assigned to Ikegai Tekko Kabushiki Kaisha. Invention is credited to Mamoru Takiura.
United States Patent |
3,876,736 |
Takiura |
April 8, 1975 |
METHOD OF MOLDING SYNTHETIC RESINS THROUGH HIGH-PRESSURE FLUID
CROSS-LINKING PROCESS AND RELEVANT APPARATUS
Abstract
A method of molding a synthetic resin which comprises: kneading
a mixture obtained by dispersing a cross-linking agent in said
synthetic resin, while melting said resin, by applying such
temperature and pressure as will not give rise to a cross-linking
reaction, introducing the thus kneaded mixture into plural reaction
chambers to effect cross-linking therein by applying an appropriate
temperature and high pressure, and extruding the thus cross-linked
material continuously from said reaction chambers through a head
die for molding same; and a molding apparatus relevant to said
method.
Inventors: |
Takiura; Mamoru (Kawasaki,
JA) |
Assignee: |
Ikegai Tekko Kabushiki Kaisha
(Tokyo, JA)
|
Family
ID: |
13587129 |
Appl.
No.: |
05/289,993 |
Filed: |
September 18, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Sep 30, 1971 [JA] |
|
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46-75818 |
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Current U.S.
Class: |
264/40.7;
264/211.24; 264/328.19; 264/349; 425/145; 264/211; 264/328.13;
264/328.6; 425/378.1 |
Current CPC
Class: |
B29C
45/54 (20130101); B29C 45/76 (20130101); B29C
2045/545 (20130101) |
Current International
Class: |
B29C
45/54 (20060101); B29C 45/53 (20060101); B29C
45/76 (20060101); B29g 002/00 () |
Field of
Search: |
;264/40,176R,328,174,329,211,349,99,25
;425/145,378,379,204,243,244 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Thurlow; Jeffery R.
Attorney, Agent or Firm: Woodhams, Blanchard & Flynn
Claims
What is claimed is:
1. A method of cross-linking and extrusion molding a cross-linkable
resin selected from the group consisting of polyolefin resin and
polyvinyl chloride resin, comprising repeatedly carrying out the
cycle of steps of:
1. within a main reaction chamber having a rotatable and axially
reciprocatable plasticizing screw therein, feeding said resin and
an amount of cross-linking agent capable of cross-linking said
resin into said main reaction chamber and therein rotating said
screw and simultaneously imposing a back pressure on said screw to
resist retracting axial movement thereof so that rotation of said
screw melts and kneads said resin and distributes the cross-linking
agent uniformly therein to form a plasticized mixture thereof and
forwards the plasticized mixture into the forward end of said main
reaction chamber, and the accumulation of said plasticized mixture
in said forward end of said main reaction chamber forces said screw
to retract in said main reaction chamber against said back
pressure, stopping the rotation of said screw when said screw has
retracted to a predetermined retracted position in said main
reaction chamber and continuing to apply said back pressure on said
screw whereby a charge of plasticized material is present in the
forward end of said main reaction chamber and is subjected to said
back pressure, the temperature and pressure applied on said
plasticized mixture in the main reaction chamber being insufficient
to cause the cross-linking reaction to occur so that the charge of
plasticized material is not cross-linked;
2. simultaneously with the retracting movement of said screw,
advancing at a constant maximum speed a reciprocatable first
plunger in a first secondary reaction chamber which is isolated
from said main reaction chamber and is filled with a charge of
previously plasticized material undergoing cross-linking to
discharge the latter charge from said first secondary reaction
chamber through a transfer valve and thence through an extrusion
die to form an extrusion molding;
3. while said first plunger is still advancing at said constant
speed, advancing said screw axially in said main reaction chamber
to discharge therefrom the first-mentioned charge of plasticized
material and feeding said first-mentioned charge of plasticized
material through said transfer valve into a second secondary
reaction chamber which is isolated from said extrusion die and has
a reciprocatable second plunger therein and located close to the
forward end of said second secondary reaction chamber, and
simultaneously applying a second back pressure on said second
plunger less than the pressure applied to advance said screw so
that the plasticized material from said main reaction chamber
forces said second plunger to retract against said second back
pressure and fills said second secondary reaction chamber and is
continuously subjected to pressure, and terminating advancing
movement of said screw when it reaches a predetermined advanced
position in said main reaction chamber;
4. when said first plunger reaches a predetermined intermediate
advanced position in said first secondary reaction chamber,
reducing the speed of advancing movement of said first plunger from
said constant maximum speed toward zero, and simultaneously
initiating advancing movement of said second plunger at a speed
increasing from zero toward said constant maximum speed, and
simultaneously shifting said transfer valve to place both of said
first and second secondary reaction chambers in communication with
said die whereby the extrusion molding continuously being formed at
this time consists of plasticized material supplied from both of
said secondary reaction chambers, the speeds of the first and
second plungers being coordinated with each other so that the total
amount of plasticized material extruded through the die per unit
time is substantially equal to the amount extruded during said
constant maximum speed advance of said first plunger;
5. when said first plunger reaches a predetermined final advanced
position in said first secondary reaction chamber, stopping
advancing movement of said first plunger, and simultaneously
establishing advancing movement of said second plunger in said
second secondary reaction chamber at said constant maximum speed,
and simultaneously shifting said transfer valve to place said
second secondary reaction chamber in communication with said
extrusion die and out of communication with said main reaction
chamber and to place said first secondary reaction chamber in
communication with said main reaction chamber and out of
communication with said extrusion die, whereby the extrusion
molding continues to be formed by the plasticized material from
said second secondary reaction chamber; and
6. then restarting rotation of the screw and repeating steps 1 to 5
with the operations of the first and second cylinders being
reversed.
Description
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to an improvement in the method of
producing uniform molded articles from a mixture of a synthetic
resin and a cross-linking agent, as well as a molding apparatus
pertaining to said method.
B. Description of the Prior Art
Cross-linking of a polyolefin resin, such as polyethylene, is
usually performed by a procedure whereby a mixture obtained by
uniformly dispersing a cross-linking agent in said polyethylene is
first made into molded articles by means of an extrusion-molding
machine, followed by a cross-linking process. As widely used known
cross-linking processes, there are the high-pressure steam process,
the high-temperature liquid process, the lead-encasing
vulcanization process, the Grace's process, radiation process,
etc.; each process having merits and demerits. In addition, there
has recently been developed a new cross-linking process called
Engel's process. According to Engel's process, a cross-linking
agent is mixed with a thermoplastic material within a screw-driven
extruding machine, the resulting mixture is introduced into a
pressure chamber to be compressed under a pressure of 2,000
Kg/cm.sup.2 instantaneously to effect uniform dispersion of said
cross-linking agent without giving rise to the cross-linking
reaction, and then the thus treated mixture is passed through a die
held at a temperature high enough to effect the cross-linking
reaction to thereby accomplish the desired cross-linking. An object
of this process is to achieve uniform dispersion of the
cross-linking agent by means of said pressure chamber, and
cross-linking by heating from the outside at the time of passing
the mixture through said die. It is technically based on the
conception that said thermoplastic material accquires a heat curing
property after completion of the cross-linking reaction and is no
longer suitable for molding or drawing. Consequently, molding by
this process must be performed just before the material becomes
unsuitable for molding or drawing and is therefore very
unstable.
SUMMARY OF THE INVENTION
The present invention relates to a method of extruding a synthetic
resin, such as polyolefin resin, polyvinyl chloride, etc., which
comprises: kneading by means of a screw a mixture obtained by
uniformly dispersing a cross-linking agent in said resin during
melting of said material by applying such temperature and pressure
as will not give rise to the cross-linking reaction, introducing
the thus kneaded mixture into the reaction chamber to effect
cross-linking therein by applying an appropriate temperature and
high pressure, and extruding the thus cross-linked material
continuously through a head die for molding purposes; and a molding
apparatus relevant to said method. In this connection, the present
apparatus is also applicable in the production of foamy moldings
from a mixture of a synthetic resin and a foaming agent such as
organic peroxides and the like.
A factor requisite for satisfactory realization of the function of
an apparatus of this kind is to secure uniformity in size of the
extruded moldings at the time when the material held in the main
reaction chamber upon plasticizing by the screw is introduced into
the plunger-type secondary reaction chamber and further extruded
into the molding head die. And, in case of an apparatus employing a
plural number of plungers, a particularly important thing is
whether a change in size can be prevented at the time of switching
the extrusion from one plunger to another. In other words, the
operation is accompanied by various risks, such that: if it is so
devised as to set one plunger in motion after another plunger has
come to a halt, the extruded moldings will have distinct seams; and
if it is so devised as to put a plural number of plungers in motion
simultaneously for a certain period of time, the quantity of
material being extruded will increase particularly during said
period, resulting in a lack of uniformity in size of the moldings;
and so on.
The present invention has as an object to avoid the foregoing
troubles in the prior art. To be precise, an object is to provide a
method of working a double plunger by automatically controlling the
speed of the constituent plungers according to a prescribed
velocity program in order to maintain the volume of the material to
be extruded per unit time uniformly, and to provide a molding
apparatus characterized in that it ensures a stable extrusion of
the material in relation to the work of the transfer valves.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the accompanying drawings,
FIG. 1A is a schematic illustration of the overall structure of the
machine including the pressure-oil circuit system of a
high-pressure fluid cross-linking molding apparatus according to
the present invention.
FIG. 1B is a detail drawing showing the transfer valve.
FIG. 2 is a detail drawing showing the structure of the plunger
member as well as the transfer valve member, along with the
pressure-oil circuit system and the electric control system
therefor.
FIGS. 3A-3C are drawings showing the transverse section and
longitudinal section of the transfer valve.
FIG. 4 is a diagrammatic representation of the relation between the
stroke and the speed of the plunger.
FIG. 5 is a diagrammatic representation of the control block for
the electric and oil pressure servovalve mechanism.
Referring to FIG. 1 and FIG. 2, the numeral reference 1 denotes the
screw used for the purpose of plasticization and injection, 2
denotes the main reaction chamber to retain the plasticized
material, 3 denotes the hopper, 4 denotes the electric motor to
drive said screw, 5 denotes the reduction gear, 6 denotes the
piston to push said screw, 7 denotes the transfer valve housing, 8
denotes the transfer valve, 9 denotes the two-stage special
cylinder, 10 denotes the plunger, 11 denotes the secondary reaction
chamber, 12 denotes the oil-pressure cylinder for the plunger, 13
denotes the oil-pressure cylinder for the screw, 14 denotes the
servovalve for use in advancing the plunger P-1, 15 denotes the
servovalve for use in advancing the plunger P-2, 16 and 17 are the
solenoid valves to work when the plungers P-1 and P-2 are pushed
back by the injection screw 1 -- with the material between said
screw and the respective plunger -- under a pressure, 26 denotes
the relief valve to control the oil pressure for the circuit at
this juncture, and 24 and 25 denote the pump equipped for the
pressure-oil circuit to work said plungers and the electric motor
to drive said pump, respectively.
18 and 19 denote the solenoid valves for the two-stage special
cylinder to work the transfer valve, 20 denotes the solenoid valve
to work the piston 6 for the screw 1, 28 denotes the
flux-controlling valve for the purpose of regulating the speed of
screw when the piston 6 is pushed to advance the screw, 23 denotes
the relief valve to control the oil pressure of the circuit at this
juncture, 27 denotes the relief valve for the purpose of
controlling the backpressure when the screw makes a back stroke
while plasticizing the material, and 21 and 22 denote the pump
equipped for the pressure-oil circuit to work said injection screw
and transfer valve and the electric motor to drive said pump,
respectively.
29 denotes the molding head die, 30 denotes the moldings, 31
denotes the detector to detect the positions of the plungers P-1
and P-2 in motion, and 32 denotes the amplifier to amplify the
signal given by said detector.
34 denotes the automatic controller to instruct the motions of the
plunger, and 33 denotes the amplifier to amplify the instruction
signal directed to the servovalve.
The screw 1 is driven by the driving electric motor 4 and
plasticizes the material. The thus plasticized material is extruded
into the main reaction chamber 2, the pressure within said reaction
chamber being held at a backpressure as set by the relief valve 27,
and the screw 1 moves backward from the position of limit switch
LS-2 to the position LS-1 and comes to a halt. While the screw is
moving backward to thereby plasticize the material, the transfer
valve 8 is in the state as shown in FIG. 3 (A). In the meantime,
the plunger P-1 is advancing in accordance with the velocity
program (See FIG. 4) as set by the automatic controller 34, and the
material is being extruded by the molding head die. Subsequently,
the injection screw pushes the material held in the main reaction
chamber into the secondary reaction chamber 11 of the plunger P-2
through the transfer valve 8 by virtue of the pressure set by the
relief valve 23. On this occasion, said plunger moves backward by
virtue of the difference between the pressure set by the relief
valve 26 and the pressure set by the relief valve 23 (it is
essential that the pressure set by the relief valve 23 is higher
than the pressure set by the relief valve 26).
When the plunger is moving backward, the material is in the state
of being subjected to the pressure set by the relief valve 26.
Next, when the plunger P-1 advances to the position of the limit
switch LS-3, the speed of the plunger P-1 reduces rectilinearly
from a fixed speed v toward zero according to the signal of LS-3,
while the speed of the plunger P-2 increases rectilinearly from
zero toward a fixed speed v. By the signal of the limit switch
LS-3, the position of the transfer valve shifts to the neutral
position as shown in FIG. 3 (B). In this state, the plunger P-1
attains the speed of zero at the position of the limit switch LS-4
and comes to halt. And, by the signal of LS-4, the plunger P-2
attains a fixed speed v. By the signal of the limit switch LS-4,
the position of the transfer valve shifts to the state as shown in
FIG. 3 (C). Subsequently, the screw turns again and moves backward
from the position of the limit switch LS-2 to the position LS-1
while performing plasticization of the material.
The plungers P-1 and P-2 move in accordance with the velocity
diagram (See FIG. 4) set by the automatic controller in advance. To
be precise, when a set signal is given by the automatic controller
34, this signal is amplified by the servoamplifier 33 and enters
the servovalve. The servovalve then supplies a flux in proportion
to this signal to the oil pressure cylinder 12. In the event of the
motion of the plunger being inconsistent with the set signal, the
detector 31 detects the electric displacement and works to correct
said electric displacement for the plunger until the deviation
signal indicates zero. FIG. 5 is a block diagram to represent the
electric control system for this purpose.
The apparatus according to the present invention has such a merit
that, because of its being so devised as to control the speed of
plungers by a single automatic controller by virtue of the
servovalve and give diagonally opposite signals to a double plunger
by the set of limit switches LS-3, LS-4, and the set of limit
switches LS-5, LS-6, a regular cycle of the material can be
attained even when there is a difference in volume of the material
led into the respective secondary reaction chambers between the
plungers P-1 and P-2 (that is, when the positions to which the
plungers advance are various). Besides, inasmuch as it is so
devised as to secure the uniformity in volume of the material to be
extruded by controlling the speed by virtue of the movements of the
double plunger, it has also the merit that the transfer valve can
be of simple structure and suffices to make simple motions.
Moreover, because of separate provision of the pressure-oil circuit
for the plungers and the pressure-oil circuit for the screw and
transfer valve, it is convenient for maintenance, minimizes the
occurrence of troubles and ensures a stable operation.
* * * * *